In recent years, China has introduced policies to vigorously promote the upgrading and transformation of buses to electrification. Different from traditional buses, electric buses have their own characteristics in terms of power source, energy supply, cruising range, support facilities, and maintenance. This article mainly discusses the types, characteristics and technical performance influencing factors of electric bus batteries.
1. Types and characteristics of electric bus batteries
At present, China's electric bus batteries can be roughly divided into five categories, namely lead-acid batteries, nickel-metal hydride batteries, lithium-ion batteries, sodium-ion batteries and hydrogen fuel cells.
① Lead-acid battery
Lead-acid battery electrodes are mainly made of lead and its oxides, and the electrolyte is a sulfuric acid solution. In the discharge state of the lead-acid battery, the main component of the cathode is lead dioxide, and the main component of the anode is lead; in the charging state, the main component of the cathode and anode is lead sulfate.
Lead-acid batteries have the advantages of high open circuit voltage, low cost, reliable use, good high-current discharge performance, abundant raw materials and high lead recovery rate, which made lead-acid batteries used as electric bus batteries and electric bicycle batteries in the early days. The disadvantages of lead-acid batteries are serious pollution and low mass specific energy and volume specific energy. Depth of discharge, overcharge, high temperature, concentration of sulfuric acid and discharge current density all affect battery life.
② NiMH battery
NiMH battery is a battery with good performance. NiMH batteries are divided into high-voltage NiMH battery and low-voltage NiMH battery. The cathode active material of the Ni-MH battery is NiO electrode, the anode active material is metal hydride, also known as hydrogen storage alloy, and the electrolyte is 6mol/L potassium hydroxide solution. As an important direction of hydrogen energy application, nickel-metal hydride batteries are attracting more and more attention. The technology of Ni-MH batteries is mature, resistant to overcharge and overdischarge, better in safety, and has the characteristics of high energy and high power.
The disadvantage is that the material activity of the nickel-metal hydride battery is relatively strong, it is easy to escape, and the requirements for packaging technology are very high. At the same time, nickel-metal hydride batteries have low charge and discharge efficiency and large side reactions under high temperature conditions, which seriously affect the driving range of electric bus batteries, and the capacity density of batteries is low. It is mainly used in hybrid vehicles, and nickel-metal hydride batteries are currently facing elimination.
③ Lithium ion battery
Lithium-ion batteries are currently the dominant form of electric bus batteries. During the charging and discharging process, Li+ intercalates and deintercalates back and forth between the two electrodes. Lithium-ion battery is a representative of modern high-performance batteries, and has attracted much attention for its high specific energy, low self-discharge, long cycle life, no memory effect and environmental protection.
The types of lithium batteries mainly include ternary lithium batteries, lifepo4 batteries, lithium cobalt oxide batteries, lithium titanate batteries, and lithium manganese oxide batteries. Among them, ternary lithium battery and lifepo4 batteries battery are the two mainstream electric bus batteries at present.
Ternary lithium batteries are safer than lithium cobalt oxide batteries. The discharge platform voltage of the ternary lithium battery is significantly higher than that of the lithium iron phosphate battery. As electric bus batteries, the vehicle has strong power, that is, a large current discharge capability, and the existence of the discharge platform can prolong the discharge time. Ternary lithium batteries are suitable for vehicles that require high acceleration and climbing performance, such as off-road vehicles and sports cars. From the perspective of energy density, ternary batteries are better than lithium iron phosphate power batteries.
Lithium iron phosphate power battery has the advantages of high safety, good cycle performance, green environmental protection and low price. The disadvantages are low voltage platform, low tap density, low rate current, and poor low-temperature discharge performance, which are the main bottlenecks restricting lithium iron phosphate batteries. Most of BYD's electric bus batteries are lithium iron phosphate batteries.
Lithium cobalt oxide is the first generation of commercial lithium-ion batteries, which has many advantages, such as high specific energy, stable performance, high volume specific energy, and stable high and low temperature discharge capacity. The disadvantages are poor safety, high price and environmental pollution. The current commercialized small power lithium-ion battery material is mainly lithium cobalt oxide.
Lithium titanate is a anode material for lithium-ion batteries, and can be combined with cathode materials such as lithium manganate, ternary materials, or lithium iron phosphate to form a 2.4V or 1.9V lithium-ion secondary battery. The main advantages are good safety and stability, excellent fast charging performance, long cycle life and good wide temperature resistance. The main disadvantages are low energy density, high cost, flatulence, and reduced consistency in long-term use.
Lithium manganese oxide has a high voltage platform, high safety performance, and low price. The disadvantage is that the specific capacity is low, the cycle performance is poor, and the high temperature cycle performance is poor. In the past, most car manufacturers used lithium manganese oxide batteries as electric bus batteries.
④ Sodium ion battery
Sodium-ion battery is a new type of power battery recently developed. It mainly relies on the movement of sodium ions between the cathode and anode to work, which is similar to the working principle of lithium-ion batteries. Compared lithium vs sodium battery, sodium-ion batteries have the following advantages:
- First, the sodium salt raw material reserves are abundant and the price is low. Compared with the ternary cathode material of lithium-ion battery, the raw material cost is reduced by half by using iron-manganese-nickel-based cathode material;
- Second, due to the characteristics of sodium salt, it allows the use of low-concentration electrolyte to reduce costs;
- The third is that sodium ions do not form an alloy with aluminum, and aluminum foil can be used as a current collector for the anode, which can further reduce the cost by about 8% and reduce the weight by about 10%;
- Fourth, because the sodium-ion battery has no over-discharge characteristics, the sodium-ion battery is allowed to discharge to zero volts. The energy density of sodium-ion batteries is greater than 100Wh/kg, which is comparable to lithium iron phosphate batteries, but its cost advantage is obvious, and it is expected to replace traditional lead-acid batteries in large-scale energy storage.
⑤ Hydrogen fuel cell
Hydrogen fuel buses are currently the main form of fuel cell buses. A hydrogen fuel cell is different from an internal combustion engine in that it does not convert chemical energy into heat and then into useful work like an internal combustion engine, nor is it different from the charging and discharging of ordinary batteries, the mutual conversion process of electrical energy and chemical energy. Instead, the chemical energy of the fuel is directly converted into electrical energy, with little heat loss and high energy conversion efficiency.
The hydrogen fuel cell operation process uses the chemical reaction of hydrogen and oxygen to directly convert it into electrical energy, and only water is discharged, which is zero pollution. Generally, the fuel cell and the supercapacitor are used together to solve the fuel cell's requirement on the operating temperature. Fuel cells and supercapacitors are complementary, which is an ideal hybrid mode and one of the development directions of hybrid vehicles.
2. Influencing factors of technical performance of electric bus batteries
Battery polarization and cumulative charge and discharge determine battery life
Studies have shown that different power distributions and power durations have different effects on the capacity decay rate of power batteries. The charge-discharge polarization depth and cumulative charge-discharge capacity of power batteries are the main factors affecting battery capacity decline.
Battery charge and discharge SOC range affects battery life
Studies have shown that the use of high SOC range and low SOC range of electric bus batteries will have a greater impact on the life of lithium manganese oxide batteries. On the one hand, it is determined by the electrochemical characteristics of the working platform of the battery system itself, and on the other hand, due to the working mode of ion conduction inside the lithium-ion power battery, there must be a difference in ion concentration at both ends of the platform and a reduction in the ability to intercalate and extract lithium ions.
Charging and discharging the battery SOC in the range of 20% to 90% reduces the loss of active materials and the degree of side reactions. Therefore, the service life of the electric bus batteries can be extended by reducing the number of times the lithium-ion power battery is used in the 0%-20% range and 90%-100% range.
Battery operating temperature affects battery life
Relevant studies have shown that temperature factors have a greater impact on the power consumption of electric bus batteries, and the various systems of new energy buses are extremely sensitive to temperature changes. The vehicle has been in a high-temperature environment for a long time, and the "three-electric" system including the power battery is not conducive to the safe operation of new energy buses. When the electric air conditioning system is not turned on, the higher the temperature, the greater the power consumption, and the lower the temperature, the less power consumption.
4. Suggestions for using electric bus batteries
Based on the conclusions of relevant global studies, in order to prolong the service life of electric bus batteries, the following suggestions are proposed from the perspective of public transport companies using electric buses in operation:
- One is to reduce the use of batteries in high SOC and low SOC areas, and try to keep the SOC of electric bus batteries in the range of 20%-90%;
- The second is to ensure the stability and appropriateness of the battery operating temperature of the electric bus as far as possible, reduce the imbalance of current output, and select appropriate unit modules to supply power for different functions of the bus;
- The third is to suggest that in the season with high temperature, a cooling fan and a power battery liquid cooling device can be installed in the electric bus batteries pack, so that the battery can be in a working environment with a suitable temperature. In the season when the temperature is low, a layer of electrothermal film can be covered on the battery case to improve the efficiency and uniformity of heat conduction.
For the selection of electric bus batteries, it needs to be analyzed in combination with the natural environment, supporting facility conditions and line characteristics of the specific area where the vehicle is used. At present, the mainstream electric bus batteries include ternary lithium batteries and lithium iron phosphate batteries. Compared with lithium cobalt oxide batteries, the price of both is lower, but higher than that of lithium manganese oxide batteries. There are differences between the two in terms of battery life, safety, specific energy and charging speed.
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